HARVARD GAZETTE ARCHIVES
Water, Water Everywhere
Radio telescope finds water is common in universe
By Lee Simmons
Special to the Gazette
The universe, it seems, is full of water. That's the message being beamed to Earth from a new space-based radio telescope.
Launched into orbit on Dec. 5, 1998, the Submillimeter-Wave Astronomy Satellite (SWAS) is for the first time detecting vast amounts of water vapor hidden in the dark pockets of our galaxy.
The findings confirm what astronomers suspected, but have been unable to prove from the ground.
"It's very gratifying," says Gary Melnick of the Harvard-Smithsonian Center for Astrophysics (CfA), who heads the scientific team behind the effort. "After 20 years of guessing that nature might work this way, to finally get an instrument up into space, turn it on, point it toward these regions and see confirmation - - we're seeing water everywhere we look."
SWAS is designed to probe the cold, dark interstellar clouds in our galaxy where new stars are formed. The data it is collecting will provide crucial information about the composition and structure of these interstellar clouds and will improve our understanding of the early stages of star formation.
The results also offer exciting clues to the origin of the water in Earth's oceans and suggest that the presence of potentially life- sustaining water is not unique to our solar system.
Submillimeter Astronomy in Space
SWAS is the third scientific satellite produced by NASA's Small Explorer Program, which was established to build small, specialized spacecraft that are economical but scientifically powerful. The entire satellite weighs only 625 pounds and is controlled by an onboard computer not much different from a souped-up desktop PC.
The scientific instrument itself, however, is sophisticated. Observing radiation at submillimeter wavelengths, a frequency band between radio and infrared on the electromagnetic spectrum, is still a relatively new frontier in astronomy. It is in this band that very cold water and molecular oxygen radiate. A radio telescope must be built and calibrated to exacting tolerances to distinguish such tiny signals. No one had previously attempted to put such an instrument in space.
After 10 years of planning, construction, and waiting, the team gathered in the flight control room at NASA's Goddard Space Flight Center, outside Washington, D.C., for the launch this past December. "We were all holding our breath," admits Melnick. "The kick of a rocket launch is quite a jolt. It's like building a fine piece of hardware and then hitting it with a sledgehammer." To everyone's relief, the deployment went without a hitch and the instrument survived the trauma unscathed.
Every night, newly collected data is transmitted to the mission's Science Operations Center at the CfA on Concord Avenue, in Cambridge. There, astronomers analyze the data and select the next week's targets.
Besides Melnick, the other participating scientists at the CfA are Matthew Ashby, Ted Bergin, John Chang, Alex Dalgarno, Giovanni Fazio, Steven Kleiner, René Plume, John Stauffer, Patrick Thaddeus, Volker Tolls, Zhong Wang, and Yun-Fei Zhang. The project team also includes astronomers from Cornell, Johns Hopkins, the University of Massachusetts, the University of Cologne, and NASA.
Sifting the Stardust
The dark areas of the night sky are not mere voids, as once believed, but rather contain enormous quantities of matter in atomic and molecular form. This interstellar medium is concentrated in vast, amorphous clouds of dust and gas -- the atomic rubble, presumably, from the explosions of earlier generations of stars. It is from this primordial material that new stars and planets are formed.
These interstellar clouds are extremely tenuous: less dense than even the best vacuum that can be created on Earth. But a single cloud can span hundreds of trillions of miles and contain enough mass to create hundreds of thousands of stars like our sun.
Because the gas in these star-forming regions is so cold (typically less than 400 degrees F), it produces no visible light. These immense objects are generally invisible to even the most powerful optical telescopes. They do, however, emit low-energy radiation that can be detected by radio telescopes.
Over the past several decades, radio and infrared observations have taught us a great deal about these stellar nurseries. The earliest stages of star formation, however, remain poorly understood. Something causes these diffuse, quiescent clouds to become gravitationally unstable and begin to collapse, often fragmenting into smaller pieces in the process. The dimensions of this collapse are mind-boggling: analogous to something the size of Pennsylvania being compressed to the size of a nickel.
Compression on this scale generates tremendous heat. Unless this heat is removed, thermal pressure would eventually overwhelm the force of gravity, halting the process of star formation. Astronomers hypothesize that the presence of water, carbon monoxide, and molecular oxygen helps to cool the gas, permitting compression to continue.
Until now, however, it has been impossible to observe water and molecular oxygen in the cold interstellar medium because its emission is absorbed by the Earth's atmosphere. Only by lifting the telescope above this obscuring canopy has SWAS made it possible to measure the abundance of these molecules in interstellar space for the first time.
The Origins of Oceans
Will the interstellar water observed by SWAS someday end up in oceans like those on our own blue planet? "Nobody knows for sure where oceans come from," Melnick says. "It's believed that some portion of the Earth's water was produced at a later stage in the evolution of our solar system. But the exact percentage is unknown."
"It's also quite plausible," he continues, "that some of the gaseous water from the interstellar medium freezes onto dust grains that are carried along with the gas as clouds collapse. Those dust grains can stick to each other, and this might well be how comets are formed. Comets contain a lot of ice. The collision of a big dirty snowball like this with a young planet would certainly transport water."
SWAS is expected to observe several hundred star-forming regions in our galaxy during its 2- to 3-year life. Melnick's goal is not so much to collect information about individual sources, but to observe a large enough sample of sources to be able to make broad statements about the nature and dynamics of the interstellar medium.
"I come in every day and get excited by what we've learned the night before, but it's just adding another piece to the puzzle. Once we have enough pieces in place that we can stand back and say, 'Aha! That's the big picture -- this is how nature works when it comes to interstellar chemistry,' that's when I'll be willing to send up a flare and declare this mission a roaring success."
Copyright 1999 President and Fellows of Harvard College